1. Field of the Invention
The present invention relates to a lithium ion conductive composite electrolyte and a lithium ion secondary battery using the same.
2. Description of the Related Art
Conventionally, a non-aqueous electrolyte solution has been used for a lithium ion conductive electrolyte in a lithium ion secondary battery. Such a non-aqueous electrolyte solution has a lithium salt, such as LiPF6 and LiBF6, dissolved as a supporting salt in an organic solvent, such as propylene carbonate, ethylene carbonate, diethyl carbonate, and dimethyl carbonate. However, in a lithium ion secondary battery using such an electrolyte solution, the electrolyte solution can externally leak if the container is damaged or the like.
To prevent leakages of the electrolyte solution, a polymer gel electrolyte has been proposed as the lithium ion conductive composite electrolyte, in which the non-aqueous electrolyte solution is absorbed into an organic polymer. However, since this polymer gel electrolyte has a low strength and is pliable, it is difficult to handle during production of the lithium ion secondary battery, and the problem is that the positive electrode and the negative electrode tend to short-circuit, for example.
To resolve the above-described problems, a composite electrolyte has been proposed in which, to confer a required strength to the polymer gel electrolyte, the polymer gel electrolyte is held in a porous body that is three-dimensionally formed from electrochemically inert inorganic particles (see, for example, Japanese Patent Laid-Open No. 2000-123633). In this composite electrolyte, SiO2, MgO, Al2O3 and the like are used as the inorganic particles.
However, in such a composite electrolyte, since the inorganic particles do not have lithium ion conductivity, there is unevenness in the diffusion of the lithium ions on the negative electrode surface between the portion in which the polymer gel electrolyte is present and the portion in which the inorganic particle is present. Consequently, if a charge/discharge cycle is repeated, dendrites grow in the portion in which the polymer gel electrolyte is present, which causes the positive electrode and the negative electrode in the lithium ion secondary battery to short-circuit, so that sufficient cycle performance cannot be obtained.
One method to overcome this is to use inorganic particles having lithium ion conductivity instead of the above-described electrochemically inert inorganic particles. A known example of such inorganic particles is a glass ceramic represented by the chemical formula Li1+x−yAlxTi2−xSiyP3−yO12 (wherein 0≦x≦0.4, 0<y≦0.6) (see, for example, Japanese Patent Laid-Open No. 2001-015164).
The inorganic particles represented by the chemical formula Li1+x−yAlxTi2−xSiyP3−yO12 have a reduction potential of 1.5 V based on a Li+/Li electrode reaction potential. On the other hand, when a negative electrode formed from a high-capacity material, such as Li, Si, and Sn, is used in a lithium ion secondary battery, Li has a reduction potential of 0 V, S of 0.5 V, and Sn of 1.0 V. Specifically, since the above-described inorganic particles have a larger reduction potential than the high-capacity material, such as Li, Si, and Sn, they are more easily reduced than the high-capacity material.
Consequently, a composite electrolyte that uses inorganic particles represented by the chemical formula Li1+x−yAlxTi2−xSiyP3−yO12, suffers from the problem which is that when the charge/discharge cycle is repeated the inorganic particles are reduced, and lithium ion conductivity is lost. If the inorganic particles lose their lithium ion conductivity, as in the case of the above-described electrochemically inert inorganic particles, dendrites grow in the portion in which the polymer gel electrolyte is present, so that sufficient cycle performance cannot be obtained.
Further, as inorganic particles having lithium ion conductivity, a composite metal oxide represented by the chemical formula Li7La3Zr2O12 and having a garnet structure is known (see, for example, Murugan et al., Angew. Chem. Int. Ed. 46 (2007), pp. 7778 to 7781). This composite metal oxide could be used for the inorganic particles forming the above-described composite electrolyte.
However, this composite metal oxide represented by the chemical formula Li7La3Zr2O12 and having a garnet structure suffers from the drawback of agglomerating when forming the composite electrolyte along with the polymer gel electrolyte, so that sufficient lithium ion conductivity cannot be obtained.